Low bit-error-rate communication of data over a communications channel is an important requirement in communications systems. Fulfilling this requirement is increasingly difficult in high-speed communications systems with data rates exceeding multiple Gbits/s. In the case of an interface between semiconductor chips or dies, there is the additional problem associated with the highly frequency-selective transfer function of the communications channel. For lower speed communications systems, such as digital subscriber line (xDSL) and wireless local area network (WLAN) technologies, solutions for this problem include bit-loading and multiple-carrier communications techniques. These approaches achieve higher data rates over a communications channel with a frequency-selective transfer function through an improved utilization of the available bandwidth. In particular, the available bandwidth is divided into multiple frequency sub-bands (the multiple-carrier principle) and more information is transmitted in those sub-bands that suffer less attenuation and less information is transmitted in those sub-bands that suffer more attenuation (the bit-loading principle). There are several existing implementations of bit-loading and multiple-carrier communications techniques.
FIG. 1 illustrates one implementation of these techniques, an analog multi-tone system 100. During data transmission, respective subsets 110a-110m (collectively 110), of a data stream are converted to analog signals in digital-to-analog (D/A) converters 120a-120m (collectively 120). The analog signals are multiplied in multipliers 126a-126m (collectively 126) by sinusoids having different frequencies and phases provided by sinusoid generators 130a-130m (collectively 130) to produce respective sub-channel signals. In the frequency domain, the spectrum corresponding to the analog signals is therefore shifted to frequency sub-bands centered on the different carrier frequencies. The respective sub-channel signals are then filtered using steep band-pass filters 140a-140m (collectively 140) to ensure precise bit-loading. The respective sub-channel signals are then combined by a summing or other combining circuit 150 and transmitted through a channel 152.
FIG. 4a illustrates the magnitude 510 as a function of frequency 520 of the spectrum 500 corresponding to the respective sub-channel signals in precise bit-loading. The overlap of a first frequency band 530 and a second frequency band 540 is such that any signal loss due to the overlap is substantially less than the noise floor. For example, the amount of power in the first frequency band 530 that overlaps the second frequency band 540 may be less than 1 percent, and in many implementations is less than 0.1 percent. The amount of overlap, if any, depends on the implementation, the frequency selectivity of the channel 152 and a target data rate. In this document, frequency bands with power overlaps of less than 1 percent are considered to not overlap. In this illustration, there is a gap 550 between the first frequency band 530 and the second frequency band 540.
Referring back to FIG. 1, during data receiving, signals transmitted through the channel 152 are filtered by steep bandpass filters 154a-154m (collectively 154 ). The resulting respective sub-channel signals are multiplied in multipliers 160a-160m (collectively 160) by sinusoids having different frequencies and phases provided by sinusoid generators 170a-170m (collectively 170) to produce respective analog signals. The respective analog signals are integrated in integrators 174a-174m (collectively 174) to produce respective integrated analog signals. Conversion of the respective integrated analog signals by analog-to-digital (A/D) converters 180a-180m (collectively 180) produces respective subsets 190a-190m (collectively 190) of the data stream. When the system 100 is operating properly, subsets 190a-190m are substantially the same, and preferably substantially identical, to subsets 110a-110m. 
The analog multi-tone system 100, however, is difficult and expensive to implement in high-speed communications systems including those with data rates exceeding multiple Gbits/s. The steep bandpass filters 140 and 154 are the primary obstacles in realizing this technique at high data rates. There is a need, therefore, for an improved bit-loading technique for data transmission in high-speed communications systems.
Like reference numerals refer to corresponding parts throughout the drawings.